Conventionally, ceramic honeycomb structures have been widely used as, for example, a filter for purifying exhaust gases and a catalyst carrier, etc. In recent years, as the regulation of exhaust gas has been tightened, there have been increasing demands for a honeycomb structure, as a filter, whose partition walls are as thin as 120 μm or less so that it can achieve higher exhaust gas purifying performance. The partition walls of the honeycomb structure are expected to become thinner more and more in the future.
As a method for manufacturing such honeycomb structures, extrusion-forming has been generally used, and it is the width of the slits provided on their dies that determine the thickness of the partition walls of the honeycomb structures when manufacturing them by extrusion-forming. Thus, a variety of dies have been disclosed whose slits width is adjusted by forming coating layers on their bases.
Specifically, there is disclosed a die, as one example of conventional dies for forming honeycomb structures, whose slit width is adjusted by forming a coating layer on its base by electroless plating (JP-A-61-39167).
Further, there is disclosed another die for extrusion-forming a honeycomb structure whose resistance to abrasion is improved by forming, by chemical vapor deposition (CVD), a coating layer consisting of iron boride, chromium carbide, aluminum oxide, titanium carbide, titanium nitride, or titanium nitride carbide on it die base (JP-A-60-145804).
In the die for extrusion-forming a honeycomb structure described in the above patent document 1, the abrasion resistance of the coating layer is not necessarily sufficient to extrude ceramic materials, since the coating layer is formed by electroless nickel plating.
In the die disclosed in JP-A-60-145804, since the thickness of the coating layer formed by chemical vapor deposition (CVD) is up to about 30 μm, it is very difficult to provide slits of 120 μm or less wide, so long as the coating layer is formed by chemical vapor deposition on the die base where slits have been roughly provided by electrical discharge machining or grinding. Thus, even with this die, a honeycomb structure that complied with the recent demands could not be obtained.
In the meantime, there is disclosed in JP-A-2002-1716 a method for manufacturing a die for use in extrusion-forming of ceramic honeycomb structures which employs a die member provided with forming grooves and ceramic puddle feeding holes and includes a step of depositing a hardwearing material at least on the surface of the forming grooves by chemical vapor deposition, wherein the forming grooves of the die member are formed by grinding with a grinding stone, an electroless plating layer is formed at least on the surface of the forming grooves, and tungsten carbide as a hardwearing material is deposited on the electroless plating layer at 300° C. to 600° C.
Further, there is proposed in JP-A-10-309713 a die for use in extrusion-forming of a ceramic honeycomb structure whose base is provided with slits 30 to 200 μm wide, wherein the slit width is first roughly adjusted by forming a substrate layer on the base by electroless plating and then adjusted to be 30 to 200 μm by forming a surface layer consisting of W2C by chemical vapor deposition (CVD).
In these conventional manufacturing methods, however, what is disclosed is just to make up the surface layer of tungsten carbide or of a material containing W2C particles as a main ingredient, and they are not intended at all to make the surface layer or the hardwearing material provided on the die base a dense and uniform one.
Specifically, in the die obtained by the manufacturing method described in JP-A-2002-1716, since it is not manufactured taking into consideration the composition and particle diameter of the tungsten carbide which constitutes the hardwearing material, the speed of extrusion-forming ceramic puddle is apt to vary from part to part of the die at the time of extrusion-forming, whereby its formability is not necessarily satisfactory. Particularly when extrusion-forming a honeycomb structure whose partition wall thickness is 70 μm or less, poor forming is apt to occur, which has been a serious problem in meeting the demands for much thinner partition walls of honeycomb structures.
Further, in both the die obtained by the manufacturing method described in JP-A-2002-1716 and the die described in JP-A-10-309713, since the hardwearing material or the surface layer is made up of a material that contains a relatively large amount of W2C, their surface roughness is large and their resistance to pushing force is relatively large at the time of extrusion-forming. Thus, with these dies, the extrusion-forming speed is low, and productivity of honeycomb structures leaves room for improvement. Furthermore, there arises another problem, due to large friction produced on the surface of the dies when ceramic puddle moving through their slits, of making the hardwearing material or the surface layer susceptible to abrasion.